Conventionally, high-image quality optical systems for three CCDs have been proposed.
For example, the zoom lens disclosed in JP H6(1994)-347697A is constituted by four lens groups having a positive, a negative, a positive and a positive refractive power, as viewed from the object side, and performs zooming with the second lens group and focusing with the fourth lens group. Further, the third lens group is constituted by a single lens including an aspherical surface.
Similarly, the zoom lens disclosed in JP2000-305016A is constituted by four lens groups having a positive, a negative, a positive and a positive refractive power, as viewed from the object side, and performs zooming with the second lens group and focusing with the fourth lens group.
However, in order to ensure a back focus for inserting color separation prisms, and to shorten the focal length at the same time, it is necessary to weaken the refractive power of the third lens group. When the third lens group is constituted by a single lens as in JP H6(1994)-347697A, the curvatures of the lens surfaces are reduced with decreasing refractive power, so that it is not possible to perform the aberration correction sufficiently. Or, the curvature of the surface on the object side and that of the surface on the image side become close extremely, thus making it difficult to perform processing such as centering. In JP2000-305016A, the third lens group is constituted by two lenses, so that the processing limitations can be reduced. However, the first lens group is constituted by three single lenses and therefore is difficult to be assembled, and the second lens group is constituted by four single lenses, so that it is not possible to reduce the cost.